Optical studies on La 2− xSr xCuO 4− δ single crystals

Abstract

The infrared (IR) reflection spectra were measured on single crystals of La 2− x Sr x CuO 4− δ ( x=0, 0.09, 0.14 and 0.19) by using specially made far-IR and IR microspectrophotometers. The spectra show the Drude, the phonon and the mid-IR exciton bands. The near-IR to UV reflection spectra were also measured. The dielectric functions were calculated by Kramers-Kronig transformation of the reflection spectra. The plasma frequency of the Drude band is relatively small (4100 cm −1 for x=0.14). We found all predicted phonon bands and several other phonon bands which appeared by the lowering of the D 4h symmetry of the CuO 4 cluster. We confirmed the presence of the mid-IR exciton bands and the 1.7 eV band which appeared by the doping and assigned them to the transition from the deeply lying O 2p energy bands to the hole in the uppermost O 2p energy band. The mid-IR exciton bands show an anomalously strong intensity, which comes from the coupling of the exciton and the charge fluctuation of the electron.